This invention relates to a method for determining the stoichiometric end point of phosgenation reactions for producing polycarbonates and chloroformates. More particularly, it relates to methods for detecting the increase in the rate of heat generated at about the stoichiometric end point of reactions between phosgene and organo-hydroxy compounds.
Both polycarbonates and chloroformates are known classes of organic compounds. Polycarbonate polymers are known for their good engineering properties and inherent flame resistance. They can be obtained by reaction of aromatic dihydroxy compounds, such as bisphenol A, with phosgene. A polycarbonate for the purposes of this invention is any polymer having carbonate groups prepared from the use of phosgene. An aromatic polycarbonate has at least some of the carbonate groups attached to an aromatic nucleus. The chloroformates, in particular, oligomeric carbonate chloroformate mixtures, are prepared from phosgene and organo-hydroxy compounds. Mixtures of bischloroformate oligomers of formula I and monochloroformate oligomers of formula II are made from dihydroxy compounds. ##STR1## wherein R is a divalent aliphatic, alicyclic or aromatic radical and n is at least 1 and the number average for n is preferably less than 3. The reaction is similar to polycarbonate synthesis; however, a polycondensation catalyst is not used. Chloroformates have been shown to be useful as intermediates in the preparation of cyclic carbonate oligomers which may be converted to very high molecular weight polycarbonates as disclosed in copending application Ser. No. 704,122, filed Feb. 22, 1985 assigned to the same assignee as the present invention.
There are a number of known methods for preparing choloroformates and polycarbonates by reaction with phosgene. Each reaction is normally conducted interfacially; that is, in a mixed aqueous-organic system which results in the recovery of the product in the organic phase. For detailed descriptions of phosgenation reactions which provide polycarbonates, reference is made to the following U.S. Pat. Nos.: 3,155,683, 3,274,214, 3,386,954, 3,422,119, 4,129,574, 4,216,305, 4,197,394, 4,360,659, 4,224,434 and to the procedures described in Encyclopedia of Polymer Science and Technology, Polycarbonates (1969), Vol. 10, pp. 710-764, Interscience Publishing. For a detailed description of chloroformate synthesis, reference is made to U.S. Pat. Nos. 3,312,661, 3,959,335, 3,974,126 and 3,966,785, which prepare bischloroformate compositions by reacting a water soluble salt of an alkylidene diphenol with phosgene in an aqueous system employing an organic diluent.
Although the preparation of aromatic polycarbonates and chloroformates with phosgene is well known, until recently a substantial excess of the calculated stoichiometric amount of phosgene was added to the reaction vessel to insure that all the initially added organo-hydroxy compound started with would react.
U.S. Pat. No. 4,378,454, issued Mar. 29, 1983, discloses a method for determining the end point of the polycarbonate polymerization reaction. This method is based on the known solubility of phosgene in the organic solvent utilized. The solubilized phosgene reacts immediately with the bisphenol and, since there is insufficient bisphenol, once the stoichiometric end point is reached the additional solubilized phosgene can be detected by a phosgene color test, described more particularly in U.S. Pat. No. 4,378,454.
In U.S. Pat. No. 4,506,067, a method is described wherein the stoichiometric end point of the preparation of aromatic polycarbonate resin with phosgene is determined by the increase in phosgene gas occurring in the vapor phase of the reactor.
It has now been discovered that at or slightly after the time the end point of the aromatic polycarbonate or chloroformate preparation has been reached, there is a substantial increase in the rate of heat generated by the reaction mixture per unit of phosgene utilized. This increase in heat generation can be detected by any standard means and will signal the end point of the desired reaction. Phosgene addition can then be terminated, thereby saving the extra phosgene which would have been added to insure the achievement of the reaction end point. Reducing the amount of phosgene also reduces the amount of time for each reaction, whether batch or continuous, thus increasing the effective capacity of present plant equipment.